1.3 Chemical Bonding

Amides are commonly used in industrial processes as solvents, and there are suggestions that the smallest amide, methanamide, HCONH₂, could function in a similar manner to water as a solvent for life on other planets.

Draw the dot-and-cross diagram for methanamide, HCONH₂.

Predict and explain the H-C-N and C-N-H bond angles around the C and N atoms.

Predict the shape around each of the C and N atoms in HCONH₂.

State, with a reason, whether HCONH₂ is a polar molecule.

Fig. 3.1

A compound formed from magnesium and oxygen has a different structure to a compound formed from phosphorus and oxygen.

Predict the type of bonds that will occur in each compound. Explain your answer.

Explain why the melting point of phosphorus(III) oxide, P₄O₆, is lower than that of magnesium oxide in terms of their bonding and structure.

Phosphorus(III) oxide, P₄O₆, contains no P–P or O–O bonds.

In the P₄O₆ molecule, all oxygen atoms are bonded to two other atoms and all phosphorus atoms are bonded to three other atoms.

Sketch a structure for P₄O₆.

Explain the difference in electronegativity shown in Fig. 3.1 between magnesium and calcium.

Phosphorus reacts with chlorine to form PCl₅.

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PCl₅ is an example of a compound that exists as two structures depending on the conditions.

2PCl₅ (g) [PCl₄]⁺[PCl₆]^– (s)

Draw diagrams to suggest the shapes of [PCl₄]⁺ and [PCl₆]^–.

A student suggests that the PCl₅ molecule has sp³ hybridisation. Explain why the student is not correct.

Antimony(V) chloride is another Group 15 chloride. Under standard conditions, it is a liquid made from SbCl₅ molecules.

Explain why phosphorus(V) chloride in its solid form has a higher melting point than antimony(V) chloride.

Under the right conditions, molecules of SbCl₅ join together to form Sb₂Cl₁₀.

Complete Fig. 4.1 to show the bonding in Sb₂Cl₁₀.

The other Group 15 elements also form pentachlorides with the exception of nitrogen.

Suggest a reason why nitrogen does not form NCl₅.

Paperclips have a higher density than water. However, if a paperclip is lowered carefully onto the surface of water, the paperclip can float. When a drop of liquid soap is added to the water, the paperclip sinks.

Suggest explanations for these observations.

Ammonia is highly soluble in water. Draw diagrams to show the two ways that intermolecular forces can form between a molecule of water and a molecule of ammonia to explain this.

NH₃, HCl and F₂ all exist as gases at room temperature and pressure.

NH₃ has the highest boiling point of -33 °C .

Predict whether HCl or F₂ has the lowest boiling point and explain why there is a difference between their boiling points.

The structural formulae of water, methanol and methoxymethane, CH₃OCH₃, are given below. 

The physical properties of a covalent compound, such as its melting point, boiling point, vapour pressure, or solubility, are related to the strength of attractive forces between the molecules of that compound.

These relatively weak attractive forces are called intermolecular forces. They differ in their strength and include the following.

By using the letters A, B, or C, state the strongest intermolecular force present in each of the following compounds.

For each compound, write the answer on the dotted line:

Methanol and water are completely soluble in each other.

When equal volumes of ethoxyethane, C₂H₅OC₂H₅, and water are mixed, shaken, and then allowed to stand, two layers are formed.

Suggest why ethoxyethane does not fully dissolve in water. Explain your answer.

This question is about electronegativity. 

The electronegativities of some elements are shown in Table 3.1 below. 

Table 3.1

Define the term electronegativity. 

Use Table 3.1 to explain the trend in electronegativity across the Periodic Table.

Explain how the carbon-hydrogen bond (such as in CH₄) differs from the nitrogen-hydrogen bond (such as in NH₃) in terms of the bond polarity.

Explain, in terms of electronegativity, why the bonding in ammonia (NH₃) is covalent but the bonding in lithium chloride (LiCl) is ionic.

Both lithium and lithium chloride contain ions of lithium. However, the structure bonding and properties of these substances are very different.

State how the ions are held together in solid lithium and in solid lithium chloride.

Table 4.1 below shows the melting and boiling points of lithium and lithium chloride.

Table 4.1

Explain what can be deduced from the information in Table 4.1.

Two students, A and B, are comparing the properties of lithium and lithium chloride.

Student A states that both lithium and lithium chloride will conduct electricity, but Student B states that only lithium will conduct electricity. 

State whether Student A, Student B, or neither student is correct. Explain your answer.

Student A and B then went on to discuss the bonding in another substance, CaF₂.

Explain, in terms of electrons, how CaF₂ is formed from its atoms.

Alkenes contain a carbon to carbon double bond that consists of a σ bond and a π bond.

Complete the diagram to show the areas of electron density for each bond.
Label the σ bond and the π bond.

Use the information in Table 5.1 to explain why ethene contains stronger covalent bonds between carbons atoms than ethane.   

Table 5.1.

Bond length and bond energies can be used to compare the reactivity of covalent molecules. 

Compare the reactivity of hydrogen halides using the information in Table 5.2.

Table 5.2

The boiling points of the hydrogen halides are shown in Table 5.3.